1. Field of the Invention
The present invention relates to a controlling apparatus and a controlling method for controlling a wavelength division multiplexing (WDM) optical amplifier for collectively amplifying wavelength division multiplexed signal light beams to be used in optical communications, and particularly to a controlling apparatus and a controlling method for a wavelength division multiplexing optical amplifier, capable of reducing errors due to the affection of noise light to thereby realize a controlling operation with high precision.
2. Related Art
Attention has been recently directed to an optical wavelength division multiplexing (WDM) transmission system, as a technique adapted to drastically increase a transmission capacity per one thread of optical fiber transmission path, and as a basic technique leading to a lightwave network constitution. On the other hand, optical amplifiers are able to linearly amplify optical signals without once converting them into electrical signals, to thereby largely improve regenerating and repeating intervals. Thus, optical amplifiers are widely used particularly in a main line transmission system. Further, since the optical wavelength division multiplexing transmission system is also able to collectively amplify a plurality of optical signals, so that the advantage of optical amplifiers can be effectively utilized.
In a conventional optical transmission system adopting optical amplifiers, it is impossible to obtain transmission channel information by monitoring such as overheads of electrical signals at an optical amplifying-repeating device (linear repeating station). Thus, as alternative means therefor, there is known a method for wavelength division multiplexing optical supervisory channel signal light (hereinafter called xe2x80x9cOSC signal lightxe2x80x9d) into main signal light. This OSC signal light is generated such as by a WDM terminal equipment at a transmission side and terminated at an optical amplifying-repeating device, and necessary information is again multiplexed into the OSC signal light, to be transferred to a WDM terminal equipment at a receiving side.
Level controlling systems for optical amplifiers to be used in the aforementioned optical transmission system include one for keeping constant the output level of an optical amplifier (hereinafter called xe2x80x9cALC [automatic level control] systemxe2x80x9d) and another for keeping constant the gain (a difference between input and output levels) of an optical amplifier (hereinafter called xe2x80x9cAGC [automatic gain control] systemxe2x80x9d).
FIG. 10 is a block diagram showing an exemplary constitution of a general ALC system. In the ALC system such as shown in FIG. 10, there is monitored a portion of output signal light output from light level controlling means including such as an optical amplifier and an optical attenuator, and such as the gain of the optical amplifier and the loss of the optical attenuator are feedback controlled so as to keep constant the averaged level of the output signal light.
FIG. 11 is a block diagram showing an exemplary constitution of a general AGC system. In the AGC system such as shown in FIG. 11, there are monitored a portion of input signal light into light level controlling means and a portion of output signal light output from light level controlling means, and the light level controlling means is feedback controlled so as to keep constant the difference between the averaged levels of the input light and output light.
In case of utilizing an optical amplifier in optical wavelength division multiplexing transmission, when the number of wavelength channels is changed while keeping a reference signal in FIG. 10 at a constant value under the control of ALC system, there is caused fluctuation in wavelength channel levels as shown in FIG. 12. Namely, if the number of wavelength channels is changed to 4 channels when the system is ALC operating with a setting that the number of wavelength channels is 2 channels, the light level controlling means is feedback controlled such that the averaged level of the whole of the output signal light at 2 channels and that at 4 channels are constant because the reference signal of a comparator is constant, resulting in the respective wavelength channel levels being lowered after changing the number of wavelength channels. As such, it is also necessary in the ALC system to change the reference signal corresponding to the number of wavelength channels being used. In this respect, it is possible to obtain the information about the number of wavelength channels being used, such as via the OSC signal light transmitted from the WDM terminal equipment at the transmission side to the optical amplifying-repeating device.
Meanwhile, when the optical amplifier is AGC operating, the levels of respective wavelength channels are kept constant as shown in FIG. 13 even if the number of wavelength channels is changed. Namely, since the difference between the averaged levels of the input light and output light is kept constant even if the number of wavelength channels is changed from 2 channels to 4 channels, the respective wavelength channel levels are controlled to be constant before and after the change of the number of wavelength channels.
However, in the conventional controlling method for an optical amplifier for optical wavelength division multiplexing as described above, there has existed a problem of a controlling error due to the affection of noise light called amplified spontaneous emission (ASE) light to be caused upon linearly amplifying optical signals.
FIG. 14 shows an example of a spectrum of an optical wavelength division multiplexed transmission signal, where the number of wavelength channels is 8 channels. In FIG. 14, what can be seen at the lower side of spectrum peaks of respective wavelength channels, is ASE light caused upon linearly amplifying optical signals.
The spectrum shape of ASE light is determined by the amplification band of an optical amplifier, optical amplifiers having wider amplification bands (i.e., capable of amplification of many wavelength channels) have wider noise bands, and the ASE light is accumulated whenever the linear amplification is repeated. Concerning the optical signal to be monitored when conducting the level control of an optical amplifier, the light powers of respective wavelength channels are not individually monitored, but the whole of the light output of the optical amplifier is monitored such as by a PD (photodetector). This results in the inclusion of the aforementioned ASE light, leading to an error upon conducting the level control of wavelength channel light. The controlling error due to ASE light becomes more apparent, when an optical amplifier having a wide band is to be used at a smaller number of wavelength channels, for example, when an optical amplifier having a wavelength band corresponding to 32 channels is used at 1 channel.
Further, the purpose of the ALC operation of a WDM optical amplifier is to suppress the drift of an input signal level such as due to the loss fluctuation of a transmission path, to thereby ensure a stable transmission quality. As such, it is also desired to ALC operate the optical amplifier when a wavelength channel(s) is(are) added or subtracted. However, in the conventional controlling method, it has been required to once stop the ALC, when adding/subtracting wavelength channel(s).
Further, when an optical amplifier is AGC operating, it is required that the amplitude and speed of level fluctuation to be caused by addition or subtraction of wavelength channels are within a range of the AGC operation response, such that the change of the number of wavelength channels does not affect the light levels of respective wavelength channel lights. However, the response speed of AGC by the conventional controlling method has not been necessarily at a sufficient level.
Still referring to FIG. 14, the ratio between the level of wavelength channel light and that of the noise light is called an optical signal to noise ratio (hereinafter called OSNR) which is an important parameter in evaluating the quality of optical transmission signals. To accurately measure the OSNR, it is necessary to measure the spectrum of optical signals, which requires a spectrum analyzer having a wide dynamic range. However, from the standpoint of simplification of the constitution of an optical amplifying-repeating device, it is not necessarily advisable to provide a spectrum analyzer in each optical amplifying-repeating device for the purpose of measuring the OSNR.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore a first object of the present invention to reduce the controlling error due to the affection of noise light to thereby realize precise controlling of an optical amplifier. It is a second object to realize controlling of an optical amplifier capable of suppressing the drift of an input signal level such as due to the loss fluctuation of a transmission path, even when a wavelength channel(s) is(are) added or subtracted. It is a third object to realize controlling of an optical amplifier capable of reducing the aforementioned restriction of the AGC operation when adding or subtracting a wavelength channel(s). Further, it is a fourth object to realize a controlling method capable of readily monitoring the OSNR without measuring the spectrum of optical signals.
To achieve the above object, one aspect of the present invention provides a controlling apparatus for controlling an operation of a wavelength division multiplexing optical amplifier to collectively amplify wavelength division multiplexed signal light including a plurality of channel lights of different wavelengths, comprising: optical separating means for separating the wavelength division multiplexed signal lights into a plurality of wavelength groups; light power measuring means for measuring a light power per each of the plurality of wavelength groups separated by the optical separating means; and controlling means for controlling the operation of the wavelength division multiplexing optical amplifier based on the light power of one of the plurality of wavelength groups measured by the light power measuring means.
According to such a constitution, the wavelength division multiplexed signal light is divided into a plurality of wavelength groups, and the light power per each of the plurality of wavelength groups is measured by the light power measuring means. Further, one of the measured light powers of the respective wavelength groups is selected, and the operation of the wavelength division multiplexing optical amplifier is controlled by the controlling means based on the selected light power of the pertinent wavelength group. In this way, there is reduced the ratio of the noise light component included in the monitoring light to be used for controlling the optical amplifier, so as to reduce the controlling error due to the affection of noise light to thereby realize a controlling operation with high accuracy.
Further, the aforementioned controlling apparatus may further comprise: optical branching means for branching a portion of the wavelength division multiplexed signal light amplified by the wavelength division multiplexing optical amplifier, and for outputting the branched portion to the optical separating means; wherein the optical separating means has a transmission wavelength characteristic which periodically varies corresponding to the wavelength channel intervals of the wavelength division multiplexed signal light, and separates the portion of the wavelength division multiplexed signal light branched by the optical branching means into an even-numbered group including even-numbered channel lights and an odd-numbered group including odd-numbered channel lights; wherein the light power measuring means measures the respective light powers of the even-numbered group and the odd-numbered group separated by the optical separating means; and wherein the controlling means controls the operation of the wavelength division multiplexing optical amplifier, based on one of the light power of the even-numbered group and the light power of the odd-numbered group measured by the light power measuring means.
According to such a constitution, the optical branching means branches a portion of the wavelength division multiplexed signal light to be dealt with the wavelength division multiplexing optical amplifier, the thus branched portion of the signal light is separated into the even-numbered group and the odd-numbered group by the optical separating means, and the light powers of the respective groups are measured by the light power measuring means. Further, one of the measured light powers of the even-numbered group and odd-numbered group is selected, and the operation of the wavelength division multiplexing optical amplifier is controlled by the controlling means based on the thus selected light power of the pertinent group.
In the aforementioned controlling apparatus, the optical branching means may include an output side branching part for branching a portion of the wavelength division multiplexed signal light output from the wavelength division multiplexing optical amplifier; and the controlling means may include a constant-output-level controlling part for controlling the operation of the wavelength division multiplexing optical amplifier such that the output light level is constant, based on one of the light power of the even-numbered group and the light power of the odd-numbered group measured by the light power measuring means.
According to such a constitution, the output side branching part branches, as monitoring light, a portion of the wavelength division multiplexed signal light amplified by the wavelength division multiplexing optical amplifier, and the thus branched portion is separated by the optical separating means into the even-numbered group and odd-numbered group. Further, the wavelength division multiplexing optical amplifier is controlled to ALC operate by the constant-output-level controlling part, based on one of the light power of the even-numbered group and the light power of the odd-numbered group measured by the light power measuring means. In this way, it becomes possible to reduce the monitor level error of ALC due to the affection of noise light, thereby enabling realization of the ALC with high accuracy.
In addition, in the aforementioned controlling apparatus, the optical branching means may include an input side branching part for branching a portion of the wavelength division multiplexed signal light input into the wavelength division multiplexing optical amplifier, and an output side branching part for branching a portion of the wavelength division multiplexed signal light output from the wavelength division multiplexing optical amplifier, the optical separating means may include: an input side separating part for separating the portion of the wavelength division multiplexed signal light branched by the input side branching part into the even-numbered group and the odd-numbered group; and an output side separating part for separating the portion of the wavelength division multiplexed signal light branched by the output side branching part into the even-numbered group and the odd-numbered group, the light power measuring means may include: an input side light power measuring part for measuring the respective light powers of the even-numbered group and the odd-numbered group separated by the input side separating part; and an output side light power measuring part for measuring the respective light powers of the even-numbered group and the odd-numbered group separated by the output side separating part, and the controlling means may include a constant-gain controlling part for controlling the operation of the wavelength division multiplexing optical amplifier such that the level difference of the input light and output light of the wavelength division multiplexing optical amplifier is constant, based on one of the light powers of the even-numbered groups and the light powers of the odd-numbered groups measured by the input side light power measuring part and the output side light power measuring part, respectively.
According to such a constitution, the input light into and the output light from the wavelength division multiplexing optical amplifier are monitored, respectively, and the wavelength division multiplexing optical amplifier is controlled to AGC operate by the constant-gain controlling part based on one of the light powers of the even-numbered groups and the light powers of the odd-numbered groups as measured by the input side light power measuring part and the output side light power measuring part, respectively. In this way, it becomes possible to reduce the monitor level error of the AGC due to the affection of noise light, thereby enabling realization of the AGC with high accuracy.
The aforementioned controlling means of the controlling apparatus may include a selecting part for selecting, in accordance with a selection signal from the outside, one of the light power of the even-numbered group and the light power of the odd-numbered group measured by the light power measuring means, to control the operation of the wavelength division multiplexing optical amplifier based on the light power selected by the selecting part. Concretely, the controlling means preferably conducts the controlling, by selecting the light power of the odd-numbered group by the selecting part when the even-numbered channel is to be added or subtracted, and by selecting the light power of the even-numbered group by the selecting part when the odd-numbered channel is to be added or subtracted.
According to such a constitution, it becomes possible to select from the outside as to which of the monitoring values of the even-numbered group and odd-numbered group is to be used for controlling the optical amplifier. Further, when a wavelength channel is to be added or subtracted, there is selected the monitoring value of the wavelength group which does not include the wavelength channel to be added or subtracted. Thus, the operation of the optical amplifier can be continuously controlled without any interruption, even when adding or subtracting wavelength channels.
Another aspect of the present invention further provides a controlling apparatus for controlling an operation of a wavelength division multiplexing optical amplifier to collectively amplify wavelength division multiplexed signal light including a plurality of channel lights of different wavelengths, comprising: optical branching means for branching a portion of the wavelength division multiplexed signal light; optical separating means having a transmission wavelength characteristic which periodically varies corresponding to the wavelength channel intervals of the wavelength division multiplexed signal light, and separating the portion of the wavelength division multiplexed signal light branched by the optical branching means into a signal component including the channel lights of respective wavelengths and noise lights around the channel lights and a noise component including the noise lights existing in wavelength bands between the respective wavelength channel lights; light power measuring means capable of measuring the light power of the signal component separated by the optical separating means; and controlling means for controlling the operation of the wavelength division multiplexing optical amplifier based on the light power of the signal component measured by the light power measuring means.
According to such a constitution, the optical branching means branches a portion of the wavelength division multiplexed signal light to be dealt with the wavelength division multiplexing optical amplifier, the thus branched portion of the signal light is separated into the signal component and the noise component by the optical separating means, and at least the light power of the signal component is measured by the light power measuring means. Further, the operation of the wavelength division multiplexing optical amplifier is controlled by the controlling means based on the measured light power of the signal component. In this way, there is reduced the ratio of the noise light component included in the monitoring light to be used for controlling the optical amplifier, so as to reduce the controlling error due to the affection of noise light to thereby realize a controlling operation with high accuracy.
In the aforementioned controlling apparatus, the optical branching means may include an output side branching part for branching a portion of the wavelength division multiplexed signal light output from the wavelength division multiplexing optical amplifier; and the controlling means may include a constant-output-level controlling part for controlling the operation of the wavelength division multiplexing optical amplifier such that the output light level is constant, based on the light power of the signal component measured by the light power measuring means.
According to such a constitution, the output side branching part branches, as monitoring light, a portion of the wavelength division multiplexed signal light amplified by the wavelength division multiplexing optical amplifier, and the thus branched portion is separated by the optical separating means into the signal component and the noise component. Further, the wavelength division multiplexing optical amplifier is controlled to ALC operate by the constant-output-level controlling part based on the light power of the signal component measured by the light power measuring means. In this way, it becomes possible to reduce the monitor level error of ALC due to the affection of noise light, thereby enabling realization of the ALC with high accuracy.
In the aforementioned controlling apparatus, the optical branching means may include: an input side branching part for branching a portion of the wavelength division multiplexed signal light input into the wavelength division multiplexing optical amplifier; and an output side branching part for branching a portion of the wavelength division multiplexed signal light output from the wavelength division multiplexing optical amplifier, the optical separating means may include: an input side separating part for separating the portion of the wavelength division multiplexed signal light branched by the input side branching part into the signal component and the noise component; and an output side separating part for separating the portion of the wavelength division multiplexed signal light branched by the output side branching part into the signal component and the noise component, the light power measuring means may include: an input side light power measuring part capable of measuring the light power of the signal component separated by the input side separating part; and an output side light power measuring part capable of measuring the light power of the signal component separated by the output side separating part, and the controlling means may include a constant-gain controlling part for controlling the operation of the wavelength division multiplexing optical amplifier such that the level difference of the input light and output light of the wavelength division multiplexing optical amplifier is constant, based on the light powers of the signal components measured by the input side light power measuring part and the output side light power measuring part, respectively.
According to such a constitution, the input light into and the output light from the wavelength division multiplexing optical amplifier are monitored, respectively, and the wavelength division multiplexing optical amplifier is controlled to AGC operate by the constant-gain controlling part based on the light powers of the signal components measured by the input side light power measuring part and the output side light power measuring part, respectively. In this way, it becomes possible to reduce the monitor level error of the AGC due to the affection of noise light, thereby enabling realization of the AGC with high accuracy.
In addition, in the aforementioned controlling apparatus, the light power measuring means may be capable of measuring the light powers of the signal component and the noise component separated by the optical separating means, and the controlling apparatus may further comprise OSNR calculating means for calculating an averaged value of OSNR""s of the wavelength division multiplexed signal light, making use of the light powers of the signal component and the noise component measured by the light power measuring means, respectively, and making use of channel information concerning used wavelengths.
According to such a constitution, the averaged value of the OSNR""s of the wavelength division multiplexed signal light can be readily calculated by the OSNR calculating means, making use of the light powers of the signal component and the noise component to be monitored for controlling the optical amplifier.
Still another aspect of the present invention provides a controlling method for controlling an operation of a wavelength division multiplexing optical amplifier to collectively amplify wavelength division multiplexed signal light including a plurality of channel lights of different wavelengths, comprising: an optical separating step for separating the wavelength division multiplexed signal light into a plurality of wavelength groups; a light power measuring step for measuring a light power per each of the plurality of wavelength groups separated by the optical separating step; and a controlling step for controlling the operation of the wavelength division multiplexing optical amplifier based on the light power of one wavelength group, measured by the light power measuring step.
Yet another aspect of the present invention further provides a controlling method for controlling an operation of a wavelength division multiplexing optical amplifier to collectively amplify wavelength division multiplexed signal light including a plurality of channel lights of different wavelengths, comprising: an optical separating step for branching a portion of the wavelength division multiplexed signal light, and separating the thus branched portion into a signal component including the channel lights of respective wavelengths and noise lights around the channel lights and a noise component including the noise lights existing in wavelength bands between the respective wavelength channel lights; a light power measuring step for measuring the light power of the signal component separated by the optical separating step; and a controlling step for controlling the operation of the wavelength division multiplexing optical amplifier based on the light power of the signal component measured by the light power measuring step.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.